As methods for soldering a printed circuit board having electronic parts such as leads and terminals mounted thereon, various methods, such as a flat dip method, a dual pot method, a flow method, a wave method, a double wave method, a flow-dip method and a cascade method, are known for supplying a molten solder onto the electronic parts to be soldered and the soldering lands, that is, the regions to be soldered on the printed circuit board.
Such soldering methods are described in “Denshi Gijutsu, extra edition of the June 1981 issue (Vol. 23, No. 7, 1981)” (Document 1). In Document 1, the flow-dip method is explained to have features of both of the flow or wave method and dip method. That is, in the flow-dip method, a molten solder in a solder vessel is caused to overflow by a pump to form an overflowing wave as in the flow or wave method, and a printed circuit board is transferred and dipped in the overflowing wave to carry out soldering as in the dip method.
As described in Document 1, the flow-dip method is useful for soldering parts with long leads, because the temperature of the molten solder is stable and does not drop when the printed circuit board is brought into contact with a surface of the molten solder, and because the surface of the molten solder can be always kept clean and the surface of the overflowing wave is stable. Another reason is that even long leads of the parts do not have the possibility of contacting the edges of the discharge opening from which the molten solder overflows.
U.S. Pat. No. 4,512,510 (Document 2) discloses a method for improving the quality of soldering by the flow-dip method. In this method, when a printed circuit board is brought into contact with a surface of a molten solder, the printed circuit board is tilted so that the printed circuit board can be gradually brought into contact with the molten solder from one end to the other to permit a gas between the printed circuit board and the surface of the molten solder to escape. The printed circuit board is then moved along the surface of the molten solder to apply a kinetic pressure from the molten solder to the regions to be soldered so that the regions to be soldered can be fully wetted with the molten solder. Finally, the printed circuit board is gradually separated from the surface of the molten solder from one end to the other so that an appropriate amount of solder can remain on each region to be soldered by a peel back effect and a solder bridge cannot be formed between adjacent regions to be soldered.
Japanese Unexamined Patent Publication No. H6-198486 (Document 3) discloses a soldering method in which soldering is carried out in an atmosphere of a non-oxidizing inert gas such as nitrogen gas. In an inert gas atmosphere with a low oxygen concentration, because oxidation of the regions to be soldered can be prevented. Further, because the surface tension of the molten solder is decreased and, therefore, the wettability of the regions to be soldered to the molten solder is significantly improved, the molten solder can be supplied onto minute regions to be soldered so easily that what is called micro-soldering can be carried out. Additionally, since the amount of flux to be applied on the regions to be soldered can be significantly reduced, there is no need for washing of the printed circuit board (removal of residual flux) after soldering.
The method disclosed in Document 3 is exclusively applied to a flow method, a wave method, a double wave method or a cascade method in which the printed circuit board is transferred only linearly, since a conveyor can be easily installed in a chamber maintained in an inert gas atmosphere. Therefore, there has been no soldering apparatus employing a flow-dip method in which soldering is carried out in an inert gas atmosphere, since a complicated transfer means is required to transfer the printed circuit board not only horizontally but also vertically.
More specifically, in order to provide a complicated transfer means as described in Document 3 in a chamber maintained in an inert gas atmosphere, the chamber must have an extremely large volume. Thus, the inert gas feed rate must be significantly large to achieve an intended low oxygen concentration (1000 ppm, for example).
Even if a soldering apparatus having such a transfer means provided outside a chamber maintained in an inert gas atmosphere is realized, the chamber will have a large capacity to permit the printed circuit board to move up and down and, further, outside air will enter the chamber from where a part of the transfer means extends into the chamber. Therefore, the inert gas feed rate must be significantly large to maintain an intended oxygen concentration. A large inert gas feed rate leads to an increase in the cost of soldering.
In addition, soldering of a printed circuit board having parts with long leads mounted thereon, that is, a printed circuit board having a surface to be brought into contact with molten solder (a surface to be soldered) from which long leads protrudes cannot be carried out by the apparatus disclosed in Document 3. Soldering of such a printed circuit board cannot be carried out by a method other than a dip or flow-dip method. However, there is no soldering apparatus employing a flow-dip method which can carry out soldering in an inert gas atmosphere with a low oxygen concentration at a low cost for the same reason as described above.
When electronic parts with long leads such as connectors are soldered onto a printed circuit board which has been once subjected to reflow soldering, partial soldering is carried out by a flow-dip method using a mask plate having openings at positions corresponding to the regions to be soldered (see FIG. 5(b)). However, there is no soldering apparatus which can carry out soldering by a flow-dip method in an inert gas atmosphere with a low oxygen concentration for the same reason as described above. The soldering is carried out by a soldering iron robot while blowing nitrogen gas onto the regions to be soldered. Therefore, the soldering of such connectors yields low productivity.
British Patent No. 801510 (Document 4) discloses what is called a Sylvania system, in which solder is pumped continuously to a plurality of upwardly extending tubes and ejected therefrom. An article to be soldered is moved into position to touch the solder ejected from selected numbers of the tubes so that selected regions on the article are soldered.
The Sylvania method disclosed in Document 4 has the following drawbacks.
(1) The mechanism of the system is so complicated that adjustment of the mechanism or recovery from a failure takes a significant amount of time, resulting in low productivity.
(2) Since different types of tubes must be used for different types of printed circuit boards to be soldered and it takes long time (about 60 minutes) to exchange the tubes. Thus, the downtime of the soldering apparatus is long, resulting in low productivity.
(3) Since there are many tubes, it takes long time (about 30 minutes, for example) to clean the tubes, resulting in low productivity.
(4) The tubes are very expensive (about one million yen per one tube, for example).
(5) It is difficult to perform the soldering in an inert gas atmosphere in a simple manner.